In recent years, with the development of portable devices such as personal computers and mobile phones, there is an increasing need for batteries as their power supplies. Batteries to be used for such purposes are required to have a high energy density. Against such requirements, lithium secondary batteries are drawing attention, and active materials which have a higher capacity than conventionally are being developed for either their positive electrodes or their negative electrodes. Among others, an elemental, oxide, or alloy form of silicon (Si) or tin (Sn) is regarded as a promising negative-electrode active material which can provide a very high capacity.
However, when a negative electrode for a lithium secondary battery is constructed by using such negative-electrode active materials, there is a problem in that the negative electrode will be deformed through repetition of charging and discharging. The aforementioned negative-electrode active materials undergo significant volumetric changes when reacting with lithium ions. Therefore, at the time of charging and discharging, the negative-electrode active material will undergo significant expansion/contraction due to reactions of insertion and desorption of lithium ions with respect to the negative-electrode active material. Therefore, when charging and discharging are repeated, a large stress will occur in the negative electrode to cause strain, thus resulting in wrinkles, cuts, and the like. Moreover, when the negative electrode is strained and deformed, a space may be created between the negative electrode and the separator, so that the charging and discharging reaction may become nonuniform, thus locally deteriorating the battery characteristics. Therefore, it has been difficult to obtain a lithium secondary battery having sufficient charge-discharge cycle characteristics by using the aforementioned negative-electrode active material.
In order to solve the above problems, Patent Document 1 proposes forming a plurality of pillar-like active material particles of silicon on a current collector. With this construction, a space for alleviating the expansion stress of silicon can be guaranteed between active material particles, whereby deformation of the negative electrode can be reduced and deterioration of cycle characteristics can be reduced. Moreover, Patent Document 2 proposes an electrode structure in which, on a current collector, a plurality of pillar-like member are placed in a regular arrangement which are composed of an active material that forms an alloy with lithium. In this electrode structure, too, the pillar-like members expand so as to fill in the voids between pillar-like members, so that deterioration in electrode characteristics due to expansion stress can be reduced.
In the negative electrodes proposed in Patent Documents 1 and 2, a plurality of pillar-like active material particles or active material members are selectively formed on the current collector surface, these active material particles or active material members standing upright along the normal direction of the current collector. However, in a lithium secondary battery having such a negative electrode, a large part of the positive electrode active material layer is not opposing the active material particles or active material members composed of a negative-electrode active material, but opposes portions of the current collector surface that are not covered with the active material (which may be called “exposed portions of the current collector”). Therefore, the lithium which is supplied from the positive electrode active material layer at charging is likely to deposit at the exposed portions of the negative electrode current collector, without being occluded by the negative-electrode active material. Therefore, lithium is unlikely to be efficiently released from the negative electrode at discharging, so that the charge-discharge efficiency may be deteriorated.
On the other hand, Patent Document 3 and Patent Document 4 of the Applicant proposes, in a negative electrode in which a plurality of pillar-like active material particles of a negative-electrode active material are formed on a current collector, tilting the longitudinal direction of the active material particles relative to the normal direction of the current collector surface. With this construction, the proportion of the portions of the positive electrode active material layer that oppose the exposed portions of the current collector can be made small, thus making it possible to sufficiently utilize the positive-electrode active material and the negative-electrode active material, whereby a higher capacity retention rate than those of Patent Document 1 and Patent Document 2 can be obtained.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-303586    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-127561    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2005-196970    [Patent Document 4] pamphlet of International Publication No. 2007/015419